Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
835 views
in Technique[技术] by (71.8m points)

file io - Packing and Unpacking binary float in python

I am having some trouble with packing and unpacking of binary floats in python when doing a binary file write. Here is what I have done:

import struct

f = open('file.bin', 'wb')
value = 1.23456
data = struct.pack('f',value)
f.write(data)
f.close()

f = open('file.bin', 'rb')
print struct.unpack('f',f.read(4))
f.close()

The result I get is the following:

(1.2345600128173828,)

What is going on with the extra digits? Is this a rounding error? How does this work?

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Answer

0 votes
by (71.8m points)

On most platforms, Python floats are what C would call a double, but you wrote your data out as float instead, which has half the precision.

If you were to use double, you'd have less precision loss:

>>> data = struct.pack('d',value)
>>> struct.unpack('d',data)
(1.23456,)
>>> data = struct.pack('f',value)
>>> struct.unpack('f',data)
(1.2345600128173828,)

The float struct format offers only single precision (24 bits for the significant precision).


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...